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United States Patent |
6,246,483
|
Smith
,   et al.
|
June 12, 2001
|
Apparatus and method for shearographic inspection and non-destructive
testing of articles in a vacuum chamber
Abstract
Disclosed is an apparatus and method for inspecting or testing a sample
using shearography techniques. A laser shearing interferometer includes: a
source of coherent radiation, a line generator producing a line of
coherent radiation from said source, a line scanner scans the line of
coherent radiation over the sample, a shearing element generates two
laterally displaced images of the sample, and phase stepper or ramper
steps or ramps the phase of one of the two images. A video camera views
images of the sample and provides corresponding video output signals. An
image processor receives the video output signals and extracts therefrom
the frame rate of the camera in substantially realtime. A signal generator
provides the stepper and the line scanner, a signal substantially in phase
with the frame rate of the camera, a decoder is used for phase extraction
and a vacuum chamber contains the laser shearing interferometer and the
sample under test. A pressure control varies the pressure within the
vacuum chamber between several predetermined pressure values and a control
unit interfaces and synchronizes the processor, the signal generator, the
decoder and the pressure control.
Inventors:
|
Smith; John P (Preston, GB);
Salter; Phillip L (Bristol, GB);
Parker; Steve C J (Bristol, GB)
|
Assignee:
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BAE Systems plc (Farnborough, GB)
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Appl. No.:
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453573 |
Filed:
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December 2, 1999 |
Foreign Application Priority Data
Current U.S. Class: |
356/520; 356/35.5 |
Intern'l Class: |
G01B 009/02 |
Field of Search: |
356/353,360,35.5
73/800,802
|
References Cited
U.S. Patent Documents
4744659 | May., 1988 | Kitabayashi | 356/353.
|
5094528 | Mar., 1992 | Tyson, II et al.
| |
5146293 | Sep., 1992 | Mercer.
| |
5257088 | Oct., 1993 | Tyson, II et al. | 356/353.
|
5257089 | Oct., 1993 | Stetson | 356/353.
|
5481356 | Jan., 1996 | Pouet et al.
| |
5493398 | Feb., 1996 | Pfister | 356/360.
|
6031602 | Feb., 2000 | Parker et al. | 356/353.
|
Foreign Patent Documents |
42 31 578 A1 | Mar., 1994 | DE.
| |
2307550 | May., 1997 | GB.
| |
2324859 | Apr., 1998 | GB.
| |
WO 84/01998 | May., 1984 | WO.
| |
Other References
Speckle techniques for material testing, Huang et al, IEEE, 1995, pp 1-6.
|
Primary Examiner: Turner; Samuel A.
Attorney, Agent or Firm: Nixon & Vanderhye P.C.
Parent Case Text
This application is a continuation of PCT/GB99/02653 filed Aug. 3, 1999.
Claims
What is claimed is:
1. Apparatus for inspecting or testing a sample using shearography
techniques including:
a laser shearing interferometer including:
a source of coherent radiation,
line generating means for producing a line of coherent radiation from said
source,
line scanning means for scanning said line of coherent radiation over said
sample,
shearing element means for generating two laterally displaced images of the
sample, and
phase stepping or ramping means for stepping or ramping the phase of one of
the two images, and
a video camera for viewing images of the sample and for providing
corresponding video output signals,
an image processor operable to receive the video output signals and to
extract therefrom the frame rate of the camera in substantially realtime,
signal generating means for providing to said stepping or ramping means
and said line scanning means a signal substantially in phase with the
frame rate of said camera,
decoding means for phase extraction,
a vacuum chamber for containing at least the laser shearing interferometer
and sample,
pressure control means capable of varying the pressure within the vacuum
chamber between several pre-determined pressure values, and
control unit means for interfacing and synchronising said processor, said
signal generating means, said decoding means and said pressure control
means.
2. Apparatus for inspecting or testing a sample as claimed in claim 1 and
wherein the laser shearing interferometer is mounted on a movable gantry.
3. Apparatus for inspecting or testing a sample as claimed in claim 2 and
wherein the movement of the movable gantry is controlled by the control
unit.
4. Apparatus for inspecting or testing a sample as claimed in claim 2 and
wherein the gantry is adapted to operate in synchronisation with the
pressure control means and the laser scanning means.
5. Apparatus for inspecting or testing a sample as claimed in claim 3 and
wherein the gantry is adapted to operate in synchronization with the
pressure control means and the laser scanning means.
6. A method for inspecting or testing a sample using shearography
techniques including the steps of:
locating said sample and a laser shearing interferometer in a vacuum
chamber,
illuminating said sample by using a line scanning means to scan a line of
coherent radiation across the surface of the sample to generate, via a
shearing element means, two laterally displaced images of the sample,
phase stepping or ramping the phase of one of the two images during each of
the line scans using a stepping or ramping means,
observing and recording said phase stepped images by means of the video
camera, whilst maintaining the scanning of a line and phase stepping of
the images in synchronization with the frame scan rate of the camera,
pre-programming the control unit with a pressure versus time profile and
indicating points on said profile when images should be recorded and
processed, and
using the control means for controlling and synchronising the pressure in
the chamber, the scan rate of the line scanning means, the phase stepping
means and the image processor, to provide a substantially real time
dynamic image of the object under test on a monitor.
7. Apparatus for inspecting or testing a sample using shearography
techniques comprising:
a laser shearing interferometer, said interferometer including:
a source of coherent radiation;
a line generator producing a line of coherent radiation from said source;
a line scanner for scanning said line of coherent radiation over said
sample;
a shearing element generating two laterally displaced images of the sample,
and
a phase stepper stepping the phase of one of the two images; and
a video camera, responsive to viewed images of the sample, for providing
corresponding video output signals;
an image processor, responsive to the video output signals, extracting
therefrom the frame rate of the camera in substantially realtime;
a signal generator for providing to said stepper and said line scanner a
signal substantially in phase with the frame rate of said camera;
a decoder for phase extraction;
a vacuum chamber containing at least the laser shearing interferometer and
said sample;
a pressure control varying the pressure within the vacuum chamber between
several predetermined pressure values; and
a control unit interfacing with and synchronizing said processor, said
signal generator, said decoder and said pressure control.
8. Apparatus for inspecting or testing a sample as claimed in claim 7 and
wherein the laser shearing interferometer is mounted on a movable gantry.
9. Apparatus for inspecting or testing a sample as claimed in claim 8 and
wherein the movement of the movable gantry is controlled by the control
unit.
10. Apparatus for inspecting or testing a sample as claimed in claim 9 and
wherein the gantry is adapted to operate in synchronization with the
pressure control means and the laser scanning means.
11. Apparatus for inspecting or testing a sample as claimed in claim 8 and
wherein the gantry is adapted to operate in synchronization with the
pressure control means and the laser scanning means.
12. A method for inspecting or testing a sample using shearography
techniques in a vacuum chamber including the steps of:
locating said sample and a laser shearing interferometer in a vacuum
chamber,
illuminating said sample by using a line scanner to scan a line of coherent
radiation across the surface of the sample to generate two laterally
displaced images of the sample,
phase stepping the phase of one of the two images during each of the line
scans using a stepper,
observing and recording said phase stepped images by a video camera, whilst
maintaining the scanning of a line and phase stepping of the images in
synchronization with the frame scan rate of the camera,
pre-programming a vacuum chamber pressure control unit to indicate when
images should be recorded and processed, and
using the pressure control unit to control and synchronize pressure in the
chamber, the scan rate of the line scanning means, the phase stepping
means and the image processor, to provide a substantially real time
dynamic image of the object under test on a monitor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus and method for inspecting and
testing a sample, such as an aircraft skin panel, by optical metrology and
is particularly, but not exclusively, concerned with such method and
apparatus applicable to optical non-destructive testing by shearography
for aerospace components.
2. Discussion of Prior Art
Coherent optical techniques such as holography, interferometry, electronic
speckle pattern interferometry (ESPI), speckle interferometry, particle
image velocimetry (PIV) and shearography are currently being utilised for
applications such as non-destructive testing (NDT), vibration analysis;
object contouring, stress and strain measurement, fatigue testing,
deformation analysis and fluid flow diagnosis. All these techniques have
associated drawbacks with performance being to some extent a trade-off
against specific disadvantages inherent in the individual techniques.
For example shearograpy has high sensitivity and tolerance to environmental
noise but known NDT techniques involving shearography are of limited
application because of difficulties in inspecting large areas due to
inefficiencies in the laser power available and optical beam expansion and
delivery systems. Additional problems are encountered with a relatively
low signal to noise ratio.
A shearography system works by generating two laterally displaced images of
a test sample. In practice this is achieved using a shearing element of
which there are many variants, and imaging optics. When the sample is
illuminated using coherent radiation such as visible radiation from a
laser, these twin images are modulated by a speckled pattern due to the
high coherence of the light. These two images interfere to form a
macroscopic speckle pattern, which may be recorded electronically using a
charge couple device (CCD) and a frame store. Interferometric images or
fringe patterns may be generated by subtracting two speckle patterns of
the sheared twin image where the second speckle pattern is recorded after
the test specimen has been subjected to a stressing force, such as
thermal, pressure or vibration. If an appropriate stressing force is
applied defects in the structure of the sample are revealed by highly
characteristic "figure of eight" fringes.
In practice the resulting fringe patterns are noisy due to spurious
intensity variations and consequently the sensitivity of the shearography
technique is reduced. Many techniques have been proposed for suppressing
such noise by extracting the phase difference between the sheared images
from the interferogrammes. One proposed technique, described in our UK
patent application number 9708651.6, involves the phase stepping of the
two laterally displaced images of the sample, by stepping the phase of one
of the two images by 2.pi./3 during each of the line scans of a camera so
that successive lines are incremented in phase to encode temporally, as
well as spatially, information about the sample within a single frame. The
resulting image may be decoded by running a vertical convolution mask over
the image. This technique has the advantages of suppressing a substantial
proportion of noise whilst providing single frame analysis. A laser
shearing interferometer suitable for providing such interferogramme images
is disclosed in our UK patent application 9708651.6 and is further
described below in the context of the present invention.
In known applications of shearography for non-destructive testing, a
coherent light source is usually a split beam laser. Splitting of the beam
allows a larger area of the sample to be tested, but in expanding the beam
intensity of the light varies with radius of the circular beam. This
variation in intensity of light incident on the sample being tested
results in poor quality of the reflected images and hence poor quality
interferogrammes.
Furthermore when the sample is to be tested under stress induced by
temperature or pressure variations, the known methods generally require
that the laser beam is delivered via a fibre optic cable into a sealed
chamber containing the sample, and in which the pressure or temperature
can be varied. The laser is generally situated outside of the sealed
chamber as known lasers used in NDT are susceptible to variations in
pressure and temperature changes and may not perform to the required
standard when exposed to the temperature or pressure variations required
for stressing the sample.
It is generally less desirable to use temperature variations to induce
stress changes as it is usually difficult to keep the entire sample at the
same required temperature and yet to be able to quickly and accurately
alter the temperature in the chamber to a new value.
A known pressure variation stressing technique allows the pressure in a
chamber to cycle between two pressure levels. Speckle pattern images are
recorded at ambient pressure then again at a lower, pre-determined
pressure level. An interferometric image is then generated by subtracting
the speckle pattern images recorded at the two different pressures. This
technique however can produce poor images if the sample undergoes
vibration whilst either speckle pattern image is being recorded.
SUMMARY OF THE INVENTION
The present invention seeks to provide improved apparatus and methods for
the NDT of articles using shearography, in which the articles are subject
to pressure stressing to provide interferometric images.
According to a first aspect of the present invention, there is provided
apparatus for inspecting or testing a sample using shearography techniques
including:
a laser shearing interferometer including:
a source of coherent radiation,
line generating means for producing a line of coherent radiation from said
source,
line scanning means for scanning said line of coherent radiation over said
sample,
shearing element means for generating two laterally displaced images of the
sample, and
phase stepping or ramping means for stepping or ramping the phase of one of
the two images, and
a video camera for viewing images of the sample and for providing
corresponding video output signals,
an image processor operable to receive the video output signals and to
extract therefrom the frame rate of the camera in substantially realtime,
signal generating means for providing to said stepping or ramping means and
said line scanning means a signal substantially in phase with the frame
rate of said camera,
decoding means for phase unwrapping or extraction,
a vacuum chamber for containing at least the laser shearing interferometer
and sample,
pressure control means capable of varying the pressure within the vacuum
chamber between several pre-determined pressure values, and
control unit means for interfacing and synchronising said processor, said
signal generating means, said decoding means and said pressure control
means.
Preferably said laser shearing interferometer is mounted on a movable
gantry. Advantageously the movement of the movable gantry is controlled by
the control unit.
The gantry may be adapted to operate in synchronisation with the pressure
control means and the laser scanning means.
According to a second aspect of the present invention, there is provided a
method for inspecting or testing a sample using shearography techniques
using the apparatus described above and including the steps of:
illuminating said sample by using the line scanning means to scan a line of
coherent radiation across the surface of the sample to generate, via the
shearing element means, two laterally displaced images of the sample,
phase stepping or ramping the phase of one of the two images during each of
the line scans using the stepping or ramping means,
observing and recording said phase stepped images by means of the video
camera, whilst maintaining the scanning of a line and phase stepping of
the images in symchronisation with the frame scan rate of the camera,
pre-programming the control unit with a pressure versus time profile and
indicating point of said profile when images should be recorded and
processed, and
using the control means for controlling and synchronising the pressure in
the chamber, the scan rate of the line scanning means, the phase stepping
means and the image processor, to provide a substantially real time
dynamic image of the object under test on a monitor.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, and to show how the
same may be carried into effect reference will now be made,by way of
example, to the accompanying drawings in which:
FIG. 1 shows a diagrammatic representation of apparatus according to one
aspect of the present invention for inspecting or testing a sample by
optical metrology utilising the method of the present invention,
FIG. 2 shows a diagrammatic view to an enlarged scale of an interferometer
for use in the method and apparatus of the present invention,
FIG. 3 shows a diagrammatic representation of interfaces of apparatus
according to the present invention, and
FIG. 4 shows a graph of pressure versus time for providing test profiles.
FIG. 1 shows a vacuum chamber 1, in which is a laser shearing
interferometer system 3 and an article to be inspected 17. The laser
shearing interferometer system 3 comprises a laser 5 which is adapted to
be capable of operating in a vacuum, a movable mirror 15, a charge coupled
device (CCD) imaging camera 11 which may be used for shearography and for
electronic speckle interferometry and which is capable of providing an
output signal 21, and an interferometer 9. A line generator 7 is located
adjacent the laser 5, for generating a line of coherent radiation 19. The
line generator 7 can conveniently be a lasiris line scanner which is
commercially available.
The interferometer 9 comprises a movable mirror 33 which is attached to a
PZT piezo electric transducer 31 for providing phase stepping. The
interferometer 9 further comprises a tilted mirror 41 for enabling
shearing, and a shearing element 29. Situated outside of the vacuum
chamber 1 are a frame synchronising unit 23 for processing the output
signal 21 from the CCD camera 11, and a triggered signal generator 25 and
an amplifier 27, for providing and amplifying a signal to drive the
movable mirror 15 and the transducer 31 in synchronism with the CCD camera
11.
FIG. 2 shows the interferometer 9 in more detail. The shearing element 29
comprises two prisms 35 and 37 for receiving light reflected from the
surface of the article 17 and for generating two laterally displaced
images in the direction of the arrow E.
In operation, the article 17 is illuminated by scanning a line 19 of
coherent radiation across the surface of the article 17. The coherent
radiation is provided by the laser 5 operating via the line generator to
generate the line 19. The beam from the line generator is directed along
path A to fall on the movable mirror 15. The mirror 15 rotates backwards
and forwards, as shown by the arrow B, to scan the line 19 up and down the
article 17 in the direction of the arrow D. Light reflected from the
surface of the article 17 falls on the shearing element 29. The CCD camera
11 is operable to view the article 17 via the shearing element 29, and
provides an output signal 21. This is received by the frame
synchronisation unit 23 which extracts the frame rate of the camera 11
which it uses to synchronise the rate at which the line of coherent
radiation 19 is scanned across article 17 by the movable mirror 15. Thus
the frame synchronisation unit 23 yields frame synchronising pulses which
can be filtered to drive the movable mirror 15 directly so that the mirror
15 is synchronised to the camera output signal 21 to yield a stable image
on a monitor. The filtered synchronising pulses trigger the signal
generator 25 which provides signals which are amplified by amplifier 27
and which drives the mirror 15 and the transducer 31 in synchronism with
the camera 11. As the transducer 31 receives the output signal from the
amplifier 27 it expands or contracts accordingly, and causes the attached
mirror 33 to move, thereby providing the required phase stepping. The
resulting interferogrammes are decoded by processor using a vertical
convolution mask technique, as described in our UK patent application
number 9708651.6 and produce high quality images of the article 17 for
identifying defects for viewing on a monitor.
FIG. 3 shows a laser shearing interferometer 3 located on a gantry 39,
within a vacuum chamber 1. The laser shearing interferometer 3 produces
and scans a line 19 over the surface of article 17, as described with
reference to FIGS. 1 and 2. A vacuum pump 45 is connected to the vacuum
chamber 1 via an electronic valve 43, arrow E showing direction of
exhaust. A control unit 47, situated outside of the vacuum chamber 1
controls the movement of the gantry 39. Control unit 47 also comprises the
frame synchronising unit 23, the signal generator 25 and the amplifier 27.
The control unit 47 therefore takes the output signal 21 from the camera
11 and controls the synchronisation of the mirror 15 and the transducer
31. The number of phase steps is controlled by the number of different
signals the signal generator 25 is programmed to send to the transducer
31. In this embodiment 5 phase steps are used to provide a satisfactory
signal to noise ratio. The control unit also controls the electronic valve
43, thereby controlling the amount of air introduced to or removed from
the vacuum chamber 1, and when this airflow takes place. Thus the control
unit 47 is able to synchronise the complete operation of the NDT facility.
The control unit 47 is capable of being programmed to allow phase stepped
images to be taken at various times, at various pressures during the test.
The images are processed by the control unit and displayed on a monitor.
FIG. 4 shows a graph of pressure versus time, with two profiles F and G.
These represent different profiles programmed into the control unit, for
testing different articles or for varying the conditions under which
articles are tested. For example, profile F shows a linear correlation
between pressure and time, with the pressure being increased steadily and
the points being marked at those times and corresponding pressures at
which interferometric images are recorded. Profile G shows a rapid
increase in pressure initially, during which time images are recorded with
a very short time interval between them, followed by slower increase in
pressure, with longer intervals between recorded images.
For any profile the pressure may be lowered and or raised depending on
testing requirements. In this example when the control unit is programmed
with profile F it controls the flow of air to or from the vacuum chamber
during the time period of the profile. At each pre-determined point shown
on the profile F the control unit 47 ensures that the camera 11 records 5
phase stepped sheared images, and that the phase stepping means (the
transducer 31) and the line scanning means (the movable mirror 15) are
operating in phase with the scan rate of the camera 11 to provide stable
images to the monitor. Every set of 5 phase stepped sheared images are
combined by the control unit into a single interferometric image which is
decoded and displayed on the monitor, in real time. The second decoded
interferometric image, recorded at the second point on the profile F, is
combined in real time with the first interferometric image, to provide a
dynamic image of the article being tested. The third interferometric image
is combined with the result of a combination of the first and second
images, and so on. This allows the user to see on the monitor a real time
dynamic image of the state of the article being tested as the pressure is
varied.
Recording images at various points upon a pressure curve allows the user to
see when the object starts to react to the change in pressure, and by how
much. Having several images recorded also means that if one is inaccurate,
due to vibration of the laser or article for example, then the test will
still provide accurate data from the other points and will not have to be
repeated.
For large articles, the control unit controls the movement of the gantry on
which the laser shearing interferometer is mounted, in synchronism with
the pressure control valve and the rate of scan of the camera, so that
images of the entire article may be scanned at the same pressure. This
provides very quick and accurate means for testing large articles.
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